Method of locating defects in a high-voltage insulating tube

ABSTRACT

Defects causing backstreaming of electrons in a Van de Graaff generator are located by determining the end-point energy of the X-ray spectrum resulting from the stopping of these electrons, taking a ratio of the voltage associated with the end-point energy to the machine-generated voltage, and using this ratio as a fraction of the generator length to determine defect location.

United States Patent Gray et a1. Sept. 25, 1973 [54] METHOD OF LOCATINGDEFECTS IN A 2,347,408 4/1944 Hanson 324/52 G 0 INSULATING TUBE3,588,611 6/1971 Lambden 324/52 3,199,023 8/1965 Bhimani 324/54 [75]Inventors: Joe W. Gray; Geoffrey W. Hartnell;

James C. Legg, all of Manhattan, Kans.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

[22] Filed: Aug. 30, 1972 [211 App]. No.: 284,782

[51} Int. Cl. G0lr 31/08, G01t 1/16 [58] Field of Search 250/833 R, 302,

[56] References Cited UNITED STATES PATENTS 3,135,915 6/1964 Odok 324/54VOLTflGE SOURCE Primary Examiner-Harold A. Dixon ArwmeyRoland A.Anderson [57] ABSTRACT Defects causing backstreaming of electrons in aVan de Graaff generator are located by determining the endpoint energyof the X-ray spectrum resulting from the stopping of these electrons,taking a ratio of the voltage associated with the end-point energy tothe machinegenerated voltage, and using this ratio as a fraction of thegenerator length to determine defect location.

5 Claims, 5 Drawing Figures Pmmmsm 3.761.720

SHEET 10F 4 VOLTflGE SOURCE PATENTEUSEPZSIQB SHEET 3 BF 4 kwkk Q umMETHOD OF LOCATING DEFECTS IN A HIGH-VOLTAGE INSULATING TUBE CONTRACTUALORIGIN OF THE INVENTION The invention described herein was made in thecourse of, or under, a contract with the United States Atomic EnergyCommission.

BACKGROUND OF THE INVENTION This invention relates to generators of highd.c. voltages of types associated with particle accelerators. Suchgenerators include cascaded voltage-multiplying circuits, cascadedtransformers, and Van de Graaff generators. When voltages in the rangeof multimillions of volts are produced by such generators, it becomesnecessary to develop electrical breakdown protection for the insulatorseparating the high-voltage terminal from electrical ground. Suchprotection is customarily done by installing repeated sections, eachcomprising an insulator and a conductor. The conductors are connectedtogether by resistors of high value to form a voltage divider betweenthe high-voltage terminal and electrical ground. In this way therepeating insulators are forced to sustain the same voltage, which canbe chosen within their breakdown limits. The use of this techniqueprevents the damage that would occur through cascading of an arefollowing failure of one section for any reason and the consequentincrease in voltage across the remaining sections.

The same technique is often applied when the electrostatic generator isused as part of a particle accelerator. In the latter case, anadditional consideration enters, namely, the need to minimize collisionsof accelerating particles while they are accelerating. This need is metby an accelerating tube, the interior of which is pumped to a highvacuum. Such tubes are essentially cylindrical in structure, formed ofrepeating sections of insulators and conductors similar to those of theinsulating column. It is common practice to have the same number ofsections in the accelerating tube and the insulating column, and toconnect corresponding conductors of the two so that the same voltagedivider is used for both the tube and column. Outside the acceleratingtube, electrical breakdown of the insulating column is typicallyinhibited by operating the insulating column in a relativelyinert gassuch as nitrogen or sulfur hexafluoride at a pressure of severalatmospheres.

The maximum energy that can be obtained by particles in an electrostaticaccelerator is limited to the maximum terminal voltage that can beattained. This voltage is a critical design parameter, and itsmaintenance is critical to effective operation. It has been observedthat the breakdown voltage on acceleration tubes is decreased after thetubes have been in service for some time. This is associated with thepresence of streaming particles, either electrons'or ions. Those particles having electrical charges opposite in signto the particles beingaccelerated at that point produce backstreaming; those having the samesign are accelerated with the desired particles. Such particles mayoriginate from foreign matter deposited on an insulating section orparticles of the pressuring gas that have entered the accelerating tubethrough a leak from the pressurized section. They may also be breakdownproducts of an insulator or of the assembling cement, ionized by arcingacross a defective insulator. Whatever their source, streaming particlesplace an intolerable operating limit on an electrostatic accelerator,requiring shutdown, location of the defect, and its repair. Inparticular, electrons so produced may cause an avalanche effect. Repairis often simple, since placing an electrical short circuit across adefective section will ordinarily eliminate the source of particles andallow a return to operation. While a complete and final repair mayrequire replacement of a major portion of an accelerator tube, operationadequate for most purposes can ordinarily be achieved by this effectiveelectrical removal of a small number of sections. This is analogous tostrengthening a chain by removal of a few weak links. As an example ofthe results of such a repair, the Tandem Van de Graaff Accelerator atKansas State University, Manhattan, Kansas, has been operatedsuccessfully above its design voltage with ten sections short-circuitedout of the sections in one of the two tandem sections of theaccelerating tube.

However, to fix trouble, one must find it. The cost of an abortiveattempt to find a source of streaming particles is at least forty-eighthours of turnaround time, the loss of some filler gas, which isexpensive if sulfur hexafluoride is used, and the risk of additionalthreats to cleanliness and structural integrity whenever the pressureand vacuum system are opened to the atmosphere. Past measures havecomprised a trial-and-error process of repeatedly short-circuiting trialsections of the acceleration tube until an improvement in breakdowncharacteristics is noted. An effective means of finding such a defectwould facilitate operation of electrostatic particle accelerators.

It is an object of the present inventon to provide an improved method oflocating defects in a high-voltage insulator.

It is a further object of the present invention to provide a fast,reliable method of locating a defective section in the accelerating tubeof an electrostatic particle accelerator.

It is a further object of the present invention to provide a method oflocating a defective section in the accelerating tube of a particleaccelerator without shutting down the accelerator.

It is a further object of the present invention to provide a method ofusing X-ray detection of bremsstrahlung to identify the location of adefect in an internal insulator of an electrostatic particleaccelerator.

Other objects will become apparent in the course of a detaileddescription of the invention.

SUMMARY OF THE INVENTION The location of a defect which producesextraneous particles in a high-voltage insulator is determined byapplying a high voltage to the insulator. This produces X-rays which aredetected and displayed to provide a spectrum of photon count vs. energy.The endpoint energy of this spectrum is determined, and the ratio of thevoltage of this end-point energy to the high voltage applied to theinsulator provides a ratio which, when multiplied by the length of theinsulator, gives the distance of the defect along the insulator,measured from the point to which the extraneous particles areaccelerated. If the insulator is formed of equal sections, the ratio ismultiplied by the number of sections to give the number of the defectivesection from the point to which the extraneous particles areaccelerated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional view ofa high-voltage insulating column including an apparatus for the practiceof the present invention.

FIG. 2 is an energy spectrum of detected radiation under normal anddefect conditions.

FIG. 3 is a partial sectional view of a tandem electrostatic acceleratorcontaining an apparatus for the practice of the present invention.

FIG. 4 is an energy spectrum of detected radiation indicating thepresence of a defect.

FIG. 5 is a functional representation of an apparatus for performingcalculations for the practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In certain types of equipmentusing high-voltage insulators to maintain large d.c. potentialdifferences between two points, it has been observed that X-rays areproduced by bremsstrahlung radiation. When the insulator is operatingproperly, a certain spectrum of bremsstrahlung radiation can be obtainedon suitable equipment. Defects in the insulator produce a differentspectrum of bremsstrahlung radiation. It has been discovered that thepresence of such defects and also their location can be determined byobtaining a spectrum of bremsstrahlung radiation while the high voltageis applied to the insulator, determining the end point of this spectrum,and calculating the ratio of the voltage of the end-point energy to theapplied terminal voltage. This ratio multiplied by the length of thecolumn determines the distance of the defect along the column from thepoint to which the bremsstrahlung-producing particles are accelerated.

FIG. 1 is a partial sectional view of a high-voltage insulating column 9containing an apparatus for the practice of the present invention.Column 9 is formed by alternating insulators 14 with conductors l6.TI-Ie number of insulators 14 and conductors 16 that is required toformcolumn 9 is a function of the dimensions and the dielectric strength ofinsulators l4 and the terminal voltage to be applied across column 9 byvoltage source- 17. Resistors 18 form a multisection voltage divider toinsure that the high voltage applied to terminal 20 is dropped equallyacross each insulator 14. An electrical connection 22 establishes anelectrical reference to ground 24. The construction of column 9 istypical of insulating structures used in Cockroft-Walton generators, Vande Graaff generators, X-ray sources, and the like, where d.c. potentialdifferences of the order of hundreds of thousands or millions of voltsmust be maintained between a pair of terminals or between a terminal andground.

FIG. 1 also includes a radiation detector 27 posi- When column 9 isoperating properly, radiation demotor 27 will detect only the normalspectrum which contains a background count at a relatively low andconstant level at energy values above the energy associated with theapplied terminal voltage. Below this value, increasing counts result asthe energy level is reduced. The spectrum of a properly operating columnhas an endpoint voltage equal to the terminal voltage. If column 9 hasdefects which result in the production of electrons or the generation ofions, the spectrum detected by radiation detector 27 will be differentfrom the spectrum observed in normal operation. This spectrum can bedisplayed on spectrum analyzer 28 and can be interpreted, first, toindicate the existence of a defect in column 9 and second, to identifythe location of the defect in column 9. Some of the cases of defectsproducing ionizable particles that cause radiation detectable asdescribed include failure of glue or cement between an insulator 14 andan adjoining conductor 16 or a deposit of foreign matter on an insulator14. An arc across an insulator 14 which releases ions from its site orthe presence of radiation damage to the material of an insulator 14 or aconductor 16 will also produce such radiation. If column 9 is hollow asshown and if the interior 26 of column 9 is under partial vacuum, a leakmay admit particles that can be ionized and accelerated to producedetectable radiation.

FIG. 2 shows a radiation spectrum of the type detected by radiationdetector 27 and displayed on spectrum analyzer 28. In FIG. 2, counts ofphotons are plotted as a function of energy in a way that is typical ofthe output of a conventional multichannel analyzer. Lines 30, 32 and 34represent best fits to the points indicated. Line 30 represents the bestfit to background radiation, observed even in the absence of generatoroperation, representing an essentially constant level of photon countsover the observed energy range. Line 32 represents the best fit to atypical spectrum of observed bremsstrahlung radiation in a generatorthat is operating properly with a terminal voltage of 5 MV. At lowenergies, larger numbers of particles collide to produce observed countsof X-rays. As the energy increases toward a value equivalent to theapplied terminal voltage, the count of observed particles falls off,approaching an end point 36 at a value of energy corresponding to theterminal voltage of the generator. The term end point is used herein inits conventional sense to describe the intersection of a trend linerepresenting a fit to an active portion of a spectrum with a linerepresenting either an axis or a fit to a background level. Line 34represents the best fit to a spectrum in the same generator with thesame terminal voltage having a defective section. Lines 30 and 34 nowintersect at end point 38 at an energy here indicated, for example, as 3MeV, a value that is les than the energy associated with the appliedterminal voltage of 5 MV. The defect producing the radiation representedby line 34 can be located by the following procedure. Note the number ofsections of the insulating column 9, which, for purpose of illustration,we take here to be 100, and the applied terminal voltage, here, 5 MV.Note the voltage of the end point of thespectrum produced by the defect,which is 3 MV. Take the ratio of voltages, 3 MV 12-5 MV F 0.6 Multiplythis ratio by the number of sections, X 0.6 =60 This is the number ofthe defective section, counting from the location to which the particlescausing the radiation are accelerated. If, as in the usual situation,the radiation is electron bremsstrahlung, then the electrons areaccelerated toward a positive terminal and the number determined asdescribed will identify the location of the defective section from thepositive terminal. If it were determined that positive ions produced theradiation that was observed, then the count would be made from anegative terminal.

The principles of the present invention have been applied to locatedefective sections in a Tandem Van de Graaff Accelerator at Kansas StateUniversity. FIG. 3 is a partial sectional view of a typical Tandem Vande Graaff Accelerator together with an apparatus for the practice of thepresent invention. In FIG. 3, insulating column 44 is constructed ofalternately disposed insulators 46 and conductors 48. Each conductor 48is connected electrically to an accelerating tube conductor 50. Eachpair of adjacent accelerating tube conductors 50 is separated by anaccelerating tube insulator 52 and is connected electrically by resistor53. The array of conductors 50 and insulators 52 comprises acceleratingtube 54 which is seen to be in two sections separated in high-voltageterminals 56 by stripper 58. Belt 60 acquires charge from points 62which are maintained at an elevated voltage by voltage source 64. Thesign of the electric potential difference maintained by voltage source64 determines whether high-voltage terminal 56 is charged to a positiveor a negative potential by points 66 which remove charge from belt 60and conduct this charge to high-voltage terminal 56. In operation, theinterior 68 of accelerating tube 54 is normally maintained under vacuumby vacuum pump 70. Insulating column 44 is nomially maintained under apressure greater than atmospheric pressure within enclosure 72 by pump74.

Operation of the Tandem Van de Graaff Accelerator is commenced byestablishing the vacuum in the interior 68 of accelerating tube 54 andpressurizing enclosure 72. High-voltage terminal 56 is charged to a highvoltage as described by charge conveyed along belt 60. Ion source 76 isthen actuated to supply ions which are accelerated along a portion ofthe interior 68 of accelerating tube 54 to high-voltage terminal 56.Stripper 58 then reverses this state of ionization of the ions which arethen further accelerated down accelerating tube 54 to target 77.Equipment for'the practice of the present invention is X-ray detector78, which is positioned near enclosure 72 in a location which receivesradiation, and multichannel analyzer 79, which displays the spectrum ofdetected radiation on plotter 80. If there'are no defects inaccelerating tube 54, X-ray detector 78 will detect a normal spectrumcontaining a low constant level of background radiation above an energylevel equalling a number of electron volts of the same magnitude as theelectrical voltage applied to the high-voltage terminal. Below thisenergy level, the photon count of observed radiation increases withdecreasing energy level. SUch a spectrum can be obtained by placingX-raydetector 78 near enclosure 72 in any one of a number of locations.For example, X-ray detector 78 may be placed outside enclosure 72 neartarget 77 or ion source 76 or high-voltage terminal 56. When this methodwas used to locate a defective section in the Tandem Van de GraaffAccelerator at Kansas State University, X-ray detector 78 was located atthe end of enclosure 72 near ion source 76. This choice of location wasmade for operating convenience to permit personnel to remain in thevicinity of the equipment.

Radiation near high voltage terminal 56 was observed to be higher inmagnitude and would have presented a hazard to operating personnel ifX-ray detector 78 had been located outside enclosure 72 nearhigh-voltage terminal'56.

FIG. 4 is a spectrum of electron bremsstrahlung radiation produced in aTandem Van de Graaff Accelerator exhibiting a defect of the type that isdetected by the method of the present invention. If FIG. 4, line 81 isthe best fit to the points representing a count of background radiation.Line 82 is the best fit to the count representing observedbremsstrahlung radiation. End point 84 is the intersection of lines 81and 82 which occurs at end-point energy 86. Defect location is performedby taking the ratio of the voltage associated with end-point energy 86to the terminal voltage of the machine. This ratio is then multiplied bythe number of sections of the accelerator through which electrons areaccelerated to produce the bremsstrahlung radiation. For example, in thespectrum indicated In FIG. 4, the end-point energy is 3.68 MeV. Thisspectrum was obtained with a positive voltage developed on highvoltageterminal 56. Electrons, having a negative charge, will be acceleratedtoward the positive charge of high-voltage terminal 56. The acceleratoron which this spectrum was obtained contained 155 sections in eachportion of accelerating tube 54. With the spectrum of FIG. 4 obtainedwith a terminal voltage of SMV between the high-voltage terminal andeach end of the Tandem Van de Graaff Accelerator, the process ofcalculating the location of a defective section is as follows: 3.56 MV+5 MV X 155 110. This means that the defect producing the spectrum ofFIG. 4 is sections away from the high-voltage terminal. It is notpossible to determine from the spectrum whether the defect is in thedirection of ion source 76 or target 77. Visual inspection will benecessary and will readily enable determination of the location of thedefect. Remedial action them comprises placing an electricalshort-circuit between conductors 50 at the defective section.

-The method of the present invention has been described in terms oflocating defective sections because it is customary to constructhigh-voltage insulators such as those described in repeating sections tocontrol the potential difference applied across such sections. It shouldbe appreciated that an insulator constructed withoutsuch sections canalso be the subject of the method of the present invention provided theratio as determined above is applied to the length of the insulatorrather than to the number of sections. Thus, if the spectrum of FIG. 4had been determined with 5 MV applied across an insulator that was 10meters in length, the process of calculating the location of the defectwould proceed as follows: 3.56 MV +5.0 MV 0.71. This ratio, multipliedby the length of the insulator, gives 0.71 X 10 7.1 meters, the distanceof the detect from the location to which electrons are beingaccelerated.

The practice of the present invention has been described in terms ofdetecting X-ray bremsstrahlung radiation because this is the usualrepresentation associated with such a defect. The design of acceleratingtubes 54 is normally performed using optics which removes positive andnegativa ions from the interior 68 of accelerating tube 54, leaving onlyelectrons as the unwanted particles of concern. The principles of thepresent invention are unchanged, however, if the defects which aredetected by this invention produce positive ions or negative ions andthe detector is selected to detect the appropriate radiation produced bythe acceleration of such ions. The principle of operation is alsounchanged if high-voltage terminal 56 is operated at a negative voltage,resulting in accelerating of electrons away from the high-voltageterminal 56 and positive ions toward high-voltage terminal 56.

It will be appreciated that the aforedescribed steps of the presentinvention may be accomplished automatically by an apparatus asexemplified in FIG. 5. In FIG. 5, function potentiometer 105 is set toan analog of the end-point voltage and function potentiometer 110 is setto an analog of the terminal voltage. Divider 115 takes the quotient ofend-point voltage to terminal voltage. This quotient is multiplied inmultiplier 120 by the output of function potentiometer 125, which is setto an analog of the number of sections in the accelerator tube. Theoutput of multiplier 120 is then an analog to the number of thedefective section. If function potentiometer 125 is set to an analog ofthe length of the insulating tube, the output of multiplier 120 is ananalog of the distance along the tube to the defect location. Thisoutput is displayed on meter 130.

The method of the present invention has been applied with success to thelocation of defects in a Tandem Van de Graaff Accelerator such as theone shown in FIG. 3. The location was determined within a precision of:3 percent on an accelerator tube having 155 sections. This precisionwas sufficient to facilitate visual determination of the location of thedefect and to eliminate the necessity of repeated trails to locate sucha defect.

The present invention is not to be limited by the embodiments describedabove, but should be construed according to the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of determining a defect in a d.c. highvoltage electricalinsulator comprising the steps of:

l. applying a test high d.c. voltage across said insulator, which testhigh d.c. voltage produces radiation from a defect in said insulator;

2. detecting the energy spectrum of said radiation;

3. determining the voltage of the end-point energy of said spectrum; and

4. comparing said voltage of said end-point energy with said test highvoltage,

which comparison is a measure of the existence of a defect.

2. The method of claim 1 wherein the step of comparing said voltagescomprises the step of taking a ratio of said voltage of said end-pointenergy to said test high voltage, said ratio being indicative of thepresence of said detect.

3. The method of claim 2 wherein said insulator comprises a number ofinsulating sections, and comprising in addition the step of multiplyingsaid ratio by the number of said insulating sections to produce aproduct, which product is the location number of the defective sectionas counted from the electrically positive point of application of saidtest high d.c. voltage to said insulator.

4. The method of claim 2 wherein said insulator is of known length andfurther including multiplying said ratio by the length of said insulatorto produce a product, which product is a measure of the defect locationfrom the electrically positive point of application of said test highd.c. voltage to said insulator.

5. A method of determining the location of a defective insulatingsection relative the high-voltage terminal in the accelerating tube of aTandem Van de Graaff Electrostatic Accelerator having a known number ofinsulating sections as counted from the high-voltage terminal of saidaccelerator comprising the steps of:

l. applying a test positive high voltage to said highvoltage terminal ofsaid accelerator, which test high voltage causes emission of electronsfrom a defect and also causes acceleration of said electrons to generateelectron bremsstrahlung radiation; A

2. disposing an X-ray detector near said accelerator to detect saidelectron bremsstrahlung radiation and to produce therefrom an outputcomprising a count of X-ray photons as a function of energy;

3. connecting a multichannel analyzer to said X-ray detector responsiveto said output to produce an X-ray bremsstrahlung spectrum of saidradiation;

4. connecting a plotter to said analyzer to produce a graphical plot ofsaid spectrum;

5. determining the voltage of the end-point energy of said spectrum inelectron volts;

6. forming the ratio'of said voltage of said end-point energy to saidtest high voltage;

7. multiplying said ratio by said known number of insulating sections toproduce a resultant number,

which resultant number is the number locating said defective section ascounted from said high-voltage terminal.

1. A method of determining a defect in a d.c. high-voltage electricalinsulator comprising the steps of:
 1. applying a test high d.c. voltageacross said insulator, which test high d.c. voltage produces radiationfrom a defect in said insulator;
 2. detecting the energy spectrum ofsaid radiation;
 3. determining the voltage of the end-point energy ofsaid spectrum; and
 4. comparing said voltage of said end-point energywith said test high voltage, which comparison is a measure of theexistence of a defect.
 2. detecting the energy spectrum of saidradiation;
 2. The method of claim 1 wherein the step of comparing saidvoltages comprises the step of taking a ratio of said voltage of saidend-point energy to said test high voltage, said ratio being indicativeof the presence of said detect.
 2. disposing an X-ray detector near saidaccelerator to detect said electron bremsstrahlung radiation and toproduce therefrom an output comprising a count of X-ray photons as afunction of energy;
 3. connecting a multichannel analyzer to said X-raydetector responsive to said output to produce an X-ray bremsstrahlungspectrum of said radiation;
 3. The method of claim 2 wherein saidinsulator comprises a number of insulating sections, and comprising inaddition the step of multiplying said ratio by the number of saidinsulating sections to produce a product, which product is the locationnumber of the defective section as counted from the electricallypositive point of application of said test high d.c. voltage to saidinsulator.
 3. determining the voltage of the end-point energy of saidspectrum; and
 4. comparing said voltage of said end-point energy withsaid test high voltage, which comparison is a measure of the existenceof a defect.
 4. The method of claim 2 wherein said insulator is of knownlength and further including multiplying said ratio by the length ofsaid insulator to produce a product, which product is a measure of thedefect location from the electrically positive point of application ofsaid test high d.c. voltage to said insulator.
 4. connecting a plotterto said analyzer to produce a graphical plot of said spectrum; 5.determining the voltage of the end-point energy of said spectrum inelectron volts;
 5. A method of determining the location of a defectiveinsulating section relative the high-voltage terminal in theaccelerating tube of a Tandem Van de Graaff Electrostatic Acceleratorhaving a known number of insulating sections as counted from thehigh-voltage terminal of said accelerator comprising the steps of: 6.forming the ratio of said voltage of said end-point energy to said testhigh voltage;
 7. multiplying said ratio by said known number ofinsulating sections to produce a resultant number, which resultantnumber is the number locating said defective section as counted fromsaid high-voltage terminal.